Reluctance machines are electrical machines which produce torque by the tendency of a moving component of the machine to take up a position in which the reluctance of the magnetic circuit is minimized. Typically, at least one of the stator and rotor members has magnetic saliencies which are normally realized in the form of poles projecting from the member.
The switched reluctance machine is a particular form of reluctance machine which has salient poles on both stator and rotor members. In this form they are referred to as `doubly salient` machines. The torque or electrical output (depending on whether the machine is run as a motor or a generator) is controlled by a controller which regulates the period during which a stator winding is connected electrically with a source of power.
Switched reluctance motors are realized in a variety of forms. In particular, they differ in the number of stator and rotor poles on the stationary and rotating members, respectively, and in the number of independent circuits with which the controller is able separately to switch stator windings in and out of circuit. Each set of windings separately switched in and out of circuit by the controller constitute one phase of the machine. The machine may have one or more such phases.
The theory, design and operation of switched reluctance machines is well documented, for example in the book `Switched reluctance Motors and their control` by T. J. E. Miller, Clarendon Press, 1993 and the article `The Characteristics, Design and Applications of Switched Reluctance Motors and Drives` by Stephenson et al., PCIM '93, Jun. 21-24, 1993.
FIG. 1 shows a known form of switched reluctance machine. The stator has six poles (A, A', B, B', C, C') and the rotor four poles. Each stator pole has one coil wound around it. Although only two coils on stator poles A and A' are shown in FIG. 1 for the sake of clarity, it will be appreciated that a similar arrangement would be formed in respect of the other pairs of poles. Typically, the coils on diametrically opposite poles are connected together either in series or in parallel (depending on the nature of the application of the machine) to form a phase of the machine. Thus, the machine in FIG. 1 is a three-phase machine in which the windings of one phase are switchable independently from those of the other phases. When the machine is operated, each phase is normally connected to a source of electrical power through one or more electronic switches t as shown in FIG. 2. The method of operation of such a machine using the switching circuit of FIG. 2 will be well known to the skilled person and is documented in the above references.
In general, the number of poles in a stator is such that each phase has an even number of coils associated with it. In the example of FIG. 1, each of the three phases has two coils, so the machine has six stator poles. In the example of FIG. 3 each of the three phases are made up of four coils symmetrically disposed around the stator, giving a twelve-pole stator. It will be appreciated that various combinations of numbers of rotor poles and stator poles are possible. The selection of a suitable combination is a matter of design choice for a given application.
When one phase of the machine in FIG. 1 is energized by a voltage being applied to the windings of one of the phases, a magnetic field is set up in the machine. This is shown schematically by the arrowed broken lines in FIG. 1. The lines are a representation of the lines of magnetic flux in the machine when phase A is energized. This field pattern is known as a two-pole field pattern since the magnetic flux crosses the air gap of the machine in two principal places.
Generally, when one phase of the machine of FIG. 3 is energized, a magnetic field is set up as represented by the arrowed broken lines. Such an arrangement is known as four-pole field pattern. By continuing to multiply the number of coils in one phase, field patterns with increasing numbers of poles can be produced. This can be done independently of the number of phases of the machine.
In a conventional machine having one coil on each pole, the coils are sized so that they can be assembled in turn on the poles without obstructing each other. The coils, and the gaps between poles in which they fit, are similar. The coil, when in place, cannot extend angularly past the mid-point between two adjacent stator poles as it would occupy the space available at the expense of the adjacent coil and would also impede insertion of the adjacent coil into its space. Thus, the cross sectional area of a coil side must occupy something less than half the total available cross sectional area between radially projecting adjacent poles. While the machine designer would often wish to make the coil bigger by increasing the cross sectional area to reduce current density and the consequent power loss in the coil, it is not possible to do this without increasing the overall size of the machine.
It is an object of the present invention to provide a reluctance machine structure that allows a larger coil size to be used in relation to a given pole.
It is a further object of the invention to provide a reluctance machine that is easier and cheaper to construct than known reluctance machines.